1. Field of the Invention
The present invention relates to a resin-sealed semiconductor device capable of improving in heat radiation characteristics of resin-sealed semiconductor elements.
2. Description of the Related Art
In a resin-sealed semiconductor device wherein a semiconductor chip is sealed with resin, the chip can be protected from dust and the like. In another resin-sealed semiconductor device wherein a large-sized semiconductor chip is sealed with resin at high power, the heat radiation characteristics of the chip can be enhanced using a heat radiating plate.
FIG. 1 is a cross-sectional view of a prior art resin-sealed semiconductor device capable of improving in heat radiation characteristics. In this device, a semiconductor chip 1 is mounted on a bed (heat radiation plate) 3 by die bonding paste 2. An inner lead 5a of each of lead frames 5 is adhered onto the bed 3 by an insulation tape 4. The bed 3 and lead frames 5 are formed of copper (Cu) having high thermal conductivity. For the insulation tape 4, a thermosetting tape or a thermoplastic tape having a high glass transition temperature is used.
Electrodes 1a of the semiconductor chip 1 are bonded to the inner leads 5a of the lead frames 5 by gold (Au) wires 6, respectively. The semiconductor chip 1 including the respective bonding points, is packaged by mold resin 7. An outer leak 5b of each of the lead frames 5 is formed to have a predetermined shape after the packaging.
In the resin-sealed semiconductor device with the above constitution, the heat generated from the semiconductor chip 1 is radiated outside through the bed 3 and the lead frames 5 adhered to the bed 3. However, the semiconductor device has the following drawbacks since the inner leads 5a are adhered onto the bed 3 by their respective insulation tapes 4.
If, as shown in FIG. 2, the insulation tapes 4 are thermosetting tapes, a tape curing step of hardening the tapes by heat is required after the lead frames 5 are attached to the bed 3. During the tape curing, the tapes generate an out gas. The out gas is stuck to the inner leads 5a to remarkably decrease in bonding between the inner leads 5a and gold wires 6. The out gas can be removed from the inner leads 5a by cleaning after the tape curing; however, the number of steps is increased by one, as is the manufacturing cost accordingly.
The inner leads 5a are also contaminated with the out gas generated from the tapes in a step of curing the die bonding paste 2. However, in this case, the gas stuck to the inner leads 5a cannot be removed by cleaning in consideration of damage to the semiconductor chip 1.
If, as shown in FIG. 3, the insulation tapes 4 are thermoplastic tapes, no curing step is needed since the thermoplastic tapes are softened at a high temperature to adhere the leads frames 5 to the bed 3 and then hardened at a normal temperature to fix them to each other. The contamination of the inner leads 5a due to the outer gas is not a serious problem as compared with the case where the thermosetting tapes are employed. Consequently, when the thermoplastic tapes are used, both the curing and cleaning steps are unnecessary and thus the number of manufacturing steps can be smaller than when the thermosetting tapes are used.
Generally-used thermoplastic tapes are easily softened at a temperature required for wire bonding, so that they make the adhesion of bed 3 and inner leads 5a unstable. More specifically, at the time of wire bonding, a heat source (not shown) is placed under the bed 3, and inner leads 5a are kept at a temperature suitable for bonding the inner leads 5a to the gold wires 6. Since the thermoplastic tapes are formed immediately under the bonding points at which the inner leads 5a and gold wires 6 are bonded to each other, they are easily softened at the temperature for the bonding and thus cannot support the inner leads 5a against the force applied from above during the bonding. The deterioration in adhesion of the inner leads 5a to the bed 3 greatly lowers the bonding characteristics.
To prevent the deterioration in adhesion, it is possible to use thermoplastic tapes which are softened at a temperature higher than that required for the bonding. In this case, however, the lead frames 5 have to be adhered to the bed 3 at a very high temperature, with the result that the tapes are greatly expanded, and the lead frames are warped when they are hardened at a normal temperature.
If, furthermore, the temperature for bonding is lowered, the bonding between the inner leads 5a and gold wires 6 deteriorates.
In both the cases described above, the prior art resin-sealed semiconductor device is structurally poor in (efficiency) transmission of heat from the heat source to the bonding points. More specifically, since each insulation tape 4 is interposed between the bed 3 and inner lead 5a, the heat generated from the heat source provided under the bed 3 is hard to transmit to the inner lead 5a. Therefore, the structure of the prior art device is very disadvantageous to achievement of good bonding.